Various types of horizontal axis dryers are known, consisting substantially of a cylinder having a heated surface and within which a possibly heated rotor is rotated, this being provided with a plurality of blades the purpose of which is to keep the product to be treated in a state of agitation so that it is brought turbulently into contact with the heated surface while at the same time being advanced along the cylinder axis towards discharge, cocurrently or countercurrently fed hot gas possibly being used.
These types of conventional dryer do not always achieve satisfactory drying of all types of products, particularly thermolabile or rheologically critical products, because the thickness, turbulence, dynamic and contact conditions of the layer of product under treatment lying at the cylinder heating surface are such as not to produce correct heat transfer and in particular result in the formation of a deposit at said surface by virtue of the different treatment times which the product undergoes.
This deposit results in soiling and in particular deterioration of the product, with consequent fall-off of heat transfer.
In these known dryers the product must never be statically at rest on the hot wall and the distance therefrom of the agitation element must tend to zero in order to act on the entire layer, with the result that a further problem encountered is that the difference in thermal expansion due to the temperature difference between the cylinder and rotor results in a variation in the position of the blades relative to the heated surface of the cylinder. This obviously results in irregular or incorrect and non-constant turbulence of the product under treatment, resulting in a loss of dynamicity of the system.
A further drawback of conventional dryers is that the centrifuging of the product under treatment and its distribution over the heated surface is partial, random and limited and depends on the blade inclination, with consequent poor contact with the cylinder. Again, in conventional dryers gas-product separation takes place externally with consequent problems of cost, space requirement and handling.
An object of the present invention is to provide a continuous dryer in which the drawbacks encountered in conventional continuous dryers are overcome.
This and further objects will be apparent to the expert of the art on reading the ensuing description.